U.S. patent number 4,265,250 [Application Number 06/059,643] was granted by the patent office on 1981-05-05 for electrode.
This patent grant is currently assigned to Battle Research and Development Associates. Invention is credited to Dawood Parker.
United States Patent |
4,265,250 |
Parker |
May 5, 1981 |
**Please see images for:
( Certificate of Correction ) ** |
Electrode
Abstract
An electrode for measuring gas concentration in blood and having
a disposable, skin-contact assembly installed in a reusable base;
the disposable assembly includes an anode, cathode, electrolyte,
and membrane, and the base includes a heater and means for
conducting heat and electrical current to the disposable
assembly.
Inventors: |
Parker; Dawood (El Toro,
CA) |
Assignee: |
Battle Research and Development
Associates (Cambridge, MA)
|
Family
ID: |
22024299 |
Appl.
No.: |
06/059,643 |
Filed: |
July 23, 1979 |
Current U.S.
Class: |
600/358;
204/403.02; 204/403.06; 204/415 |
Current CPC
Class: |
A61B
5/1491 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 005/00 () |
Field of
Search: |
;128/635
;204/195B,195P |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Scacci et al, "O.sub.2 Tension Monitoring", Med. Inst., vol. 10,
No. 4, pp. 192-194, Jul.-Aug. 1976. .
Vesterager, "Continuous Trans. Meas. . . PO.sub.2 ", Measurement of
O.sub.2, 1976, pp. 260-270..
|
Primary Examiner: Cohen; Lee S.
Claims
What is claimed is:
1. Apparatus for measuring the concentration of gases in blood,
comprising
a disposable skin-contact assembly, comprising
a first support,
anode and cathode electrodes mounted on said support, and
a skin-contact membrane permeable to said gases mounted on said
support adjacent to said electrodes to define an electrolyte region
between said membrane and said electrodes, and
a reusable base assembly, comprising
a second support,
a metal member mounted on said second support,
heat source means mounted on said second support for supplying heat
through said disposable assembly to said electrolyte region,
an electrical contact member mounted on said second support,
and
means for connecting said metal member and said contact member to
an electrical cable to provide an electric potential
therebetween,
said assemblies further comprising respective interlocking means
for bringing said metal member into contact with one said electrode
and said contact member into contact with the other said
electrode.
2. The apparatus of claim 1 wherein said heat source means
comprises means to heat said metal member.
3. The apparatus of claims 1 or 2 wherein said electrode in contact
with said metal member is said anode.
4. The apparatus of claim 1 wherein said interlocking means
comprises threaded portions of said assemblies, respectively.
5. The apparatus of claim 1 wherein said anode is an annular member
surrounding said cathode.
6. The apparatus of claim 1 further comprising a coating of dry
electrolyte salt in said electrolyte region, said coating of salt
being beneath said membrane, whereby said disposable assembly is
activated by applying water to the outside of said membrane, the
water penetrating the membrane and dissolving said salt coating to
form an electrolyte solution.
7. The apparatus of claim 6 wherein said membrane comprises
polyvinyl chloride or polystyrene.
8. The apparatus of claim 1 wherein said electrolyte region is free
of electrolyte prior to use of said apparatus and said membrane is
selected to pass water vapor and electrolyte ions, whereby said
disposable assembly is activated by applying an electrolyte
solution to the outside of said membrane and allowing said solution
to penetrate said membrane.
9. The apparatus of claim 8 wherein said membrane comprises
polyhydroxyethylmethacrylate.
10. The apparatus of claim 6 or 8 wherein said membrane is
deposited on said first support over said electrodes by applying a
coating of membrane material dissolved in solvent and allowing the
coating to harden.
Description
FIELD OF THE INVENTION
This invention relates to electrodes for measuring gas
concentration in blood.
BACKGROUND OF THE INVENTION
It is often desirable to measure the oxygen content of blood
without taking a blood sample (e.g., for fetal monitoring during
delivery). This can be done by measuring the oxygen released
through the skin from capillaries lying adjacent the skin surface,
the so-called transcutaneous partial pressure of oxygen (t.sub.c
PO.sub.2). Conventionally the measurement is made by adhering to
the skin an electrode having a membrane permeable to oxygen. One
such electrode is disclosed in Eberhard et al., "An Introduction to
Cutaneous Oxygen Monitoring in the Neonate," Hoffmann-La Roche
& Co., AG (1976). Oxygen passes through the membrane and into
an electrolyte region. The amount of oxygen is detected by
measuring the current flowing through the electrolyte between an
anode and a cathode. A heater in the electrode warms the skin to
stimulate release of oxygen. The anode, cathode, electrolyte
region, and heater are incorporated into a reusable unit. The
membrane is removed after each use to add fresh electrolyte. As it
is important to maintain the elevated temperature of the electrode
within a narrow temperature range to assure measurement accuracy,
the electrode is temperature calibrated.
SUMMARY OF THE INVENTION
It has been found that these electrodes can be made less expensive
and simpler to use by dividing the electrode into a reusable base
and a disposable, skin-contact assembly. The disposable assembly
includes an anode, cathode, electrolyte and membrane. The base
includes a heater and means for conducting heat and electrical
current to the disposable assembly. After each use, the disposable
assembly is removed and a new assembly installed, a much less time
consuming procedure than removing the membrane, adding electrolyte,
and calibrating. The reusable base need be temperature calibrated
only once, as it contains the heater.
In some preferred embodiments, the membrane is dip-coated onto the
electrolyte region by applying a solution of the membrane dissolved
in a solvent and allowing it to harden; in use the membrane is
activated by applying a drop of electrolyte to the outside of the
membrane and waiting for the electrolyte to penetrate; heat and
electricity are conducted to the disposable assembly via its
threaded connection to the base and an annular metallic (e.g.,
silver) member forming the anode; the cathode is a rod (e.g.,
platinum) mounted within and insulated (e.g., by a glass annulus)
from the annular anode; electrical current reaches the cathode via
a resilient contact in the base; the membrane is
polyhydroxyethylmethacrylate; and temperature sensors (e.g.,
thermistors) are provided in the base for controlling the heater
and for signalling an alarm if an unsafe temperature is reached.
The dip coating provides very repeatable membrane characteristics,
and thus permits calibration (zero offset and gain) for gas
concentration to be done initially and not repeated for each new
disposable assembly.
In other preferred embodiments, a coating of dry electrolyte salt
is applied before the membrane coating, and the electrode is
activated by applying a drop of water.
PREFERRED EMBODIMENTS
The structure and operation of preferred embodiments of the
invention will now be described.
DRAWING
FIG. 1 is a somewhat diagrammatic, sectional view of an electrode
embodying the invention.
FIG. 2 is a plan view of the underside of said embodiment, showing
the membrane, cathode, and anode.
FIG. 3 is a fragmentary, sectional view of another embodiment of
the invention.
STRUCTURE
Referring to FIG. 1, there is shown an electrode 10 for measuring
transcutaneous oxygen pressure (t.sub.c PO.sub.2). The electrode
consists of reusable base 12 and disposable, skin-contact assembly
11.
Base 12 has a knurled Delrin outer cover 13 (1.5 cm diameter, 0.9
cm tall) surrounding inner body 24 of stainless steel. Heating coil
14 (copper wire, 200 ohms) is wound in annular gap 15 between cover
13 and body 24. Two thermistors 16, 18 (Fenwal type, 100 K ohms at
25.degree. C.) are embedded in the bottom of body 24 next to female
threads 21, which engage mating threads on assembly 11. A
phosphor-bronze spring contact 20 extends downward in the center of
the base and is supported in epoxy potting compound 25. Wires from
cable 22 connect contact 20, body 24, and the thermistors 16, 18 to
conventional control electronics (not shown).
Disposable assembly 11 has a knurled Delrin outer cover 28 (2.0 cm
diameter) surrounding an inner silver annular member 32, the end of
which forms anode 31. Cover 28 includes an annular groove 27 and
O-ring seal 29, for sealing with base 12. Member 32 has male
threads 33 which mate with threads 21 on the base. Member 32 is
sealed with epoxy to central glass insulator 34, which in turn
surrounds and is sealed with epoxy to rod 36 (25 microns in
diameter), the end of which forms cathode 37. The skin-contacting
base of assembly 11 is dip-coated with a solution of
polyhydroxyethylmethacrylate (PHEMA) dissolved in a solvent. The
solvent evaporates to leave water-vapor permeable membrane 40
(FIGS. 1 and 2). A retaining ring 30 is then bonded over the
membrane to retain it.
OPERATION
The first step in use of the electrode is calibrating thermistor
16, heating coil 14, and the electronics so as to produce the
desired operating temperature (either 44.degree. C. or 45.degree.
C.). This calibration is done once for the base and need not be
repeated each time a new disposable assembly is inserted.
The next step is to calibrate the electronics for the zero offset
and gain relating electrical current to oxygen concentration.
Either or both of these calibrations need not be repeated each time
a new disposable assembly is inserted.
To make an oxygen measurement on the skin, a new assembly 11 is
screwed into base 12 and activated by applying a drop of
electrolyte solution (saline) on the surface of membrane 40. The
solution penetrates the membrane to form an electrolyte solution 38
between the membrane and anode 32 and cathode 36. Penetration takes
about 60 seconds for the PHEMA membrane.
Power is now applied to the heater coil, and the electrode is
allowed to reach equilibrium temperature (44.degree. C. or
45.degree. C.) while suspended in air and out of contact with the
skin. If an oxygen calibration is to be performed, the electronics
are adjusted to make the output correspond to the partial pressure
of ambient oxygen. For example, at a barometric pressure of 760 mm
Hg and an electrode temperature of 44.degree. C., the partial
pressure of O.sub.2 in air (which is 20.9% O.sub.2) is 145 mm Hg. A
further correction for the vapor pressure of water vapor is also
made because in operation the electrode is continuously in contact
with water on the skin.
The electrode is then adhered to the skin with an adhesive disk
such as those used with EKG machines. A waiting period of about 20
minutes is required to dilate the capillaries sufficiently for
release of oxygen. After measurements are complete, assembly 11 is
unscrewed from base 12 and disposed of.
Temperature is maintained at the electrolyte by heat from coil 14
conducted through steel body 24 and anode 32. The output of
thermistor 16 is used to switch current to coil 14 on and off to
maintain anode temperature to within 0.2.degree. C. of the desired
temperature. The second thermistor 18 activates a safety alarm if
an unsafe temperature is reached.
A voltage of (e.g., 800 mV) is applied between cathode 36 and anode
32. Glass insulator 34 provides a high-resistance path between
anode and cathode in order to make possible measurement of the very
small electrolyte current (about 1.times.10.sup.-9 A). The
excellent glass to cathode seal provided by the epoxy assures that
only the tip surface of the cathode is exposed to the electrolyte.
This eliminates variation in the output current due to variations
in exposed cathode surface area.
OTHER EMBODIMENTS
Other embodiments of the invention are within the following claims.
For example, a coating 100 (FIG. 3) of electrolyte salt such as KCl
dispersed in methyl cellulose can be applied before the membrane
coating and water substituted for the electrolyte solution as the
activating liquid. In this instance the membrane need only be
permeable to water vapor, and thus polyvinylchloride or polystyrene
can be substituted for the polyhydroxyethylmethacrylate. These
membrane materials require about 20 to 50 minutes for penetration
of water vapor. One method for reducing this time is to store the
disposable assemblies in humid bags.
* * * * *